This invention relates to wireless communications systems, methods and devices and, more particularly, to satellite and terrestrial wireless communications systems, methods and devices.
Cellular wireless communications systems, methods and devices are widely used for voice, multimedia and/or data communications. As is well known to those having skill in the art, cellular wireless communications systems, methods and devices include terrestrial cellular wireless communications systems, methods and devices and satellite cellular wireless communications systems, methods and devices.
In cellular wireless communications systems, methods and devices, a plurality of cells are provided, each of which can serve a portion of an overall service region, to collectively provide service to the overall service region. A wireless terminal communicates with a base station (a terrestrial base station or a satellite) over bidirectional communication pathways. Wireless communication signals are communicated from the satellite or terrestrial base station over a downlink or forward link (also referred to as a “forward service link”); and wireless communications signals are communicated from the wireless terminal to the satellite or terrestrial base station over an uplink, return link or reverse link (also referred to as a “return service link”). The overall design and operation of cellular wireless communications systems, methods and devices are well known to those having skill in the art, and need not be described further herein.
As used herein, the term “wireless terminal” includes cellular and/or satellite radiotelephones with or without a multi-line display; Personal Communications System (PCS) terminals that may combine a radiotelephone with data processing, facsimile and/or data communications capabilities; Personal Digital Assistants (PDA) or smart phones that can include a radio frequency transceiver and a pager, Internet/Intranet access, Web browser, organizer, calendar and/or a global positioning system (GPS) receiver; and/or conventional laptop (notebook) and/or palmtop (netbook) computers or other appliances, which include a radio frequency transceiver. As used herein, the term “wireless terminal” also includes any other radiating user device that may have time-varying or fixed geographic coordinates and/or may be portable, transportable, installed in a vehicle (aeronautical, maritime, or land-based) and/or situated and/or configured to operate locally and/or in a distributed fashion over one or more terrestrial and/or extra-terrestrial location(s). A wireless terminal also may be referred to herein as a “radiotelephone, a “radioterminal,” a “mobile terminal,” a “wireless user device,” a “terminal,” “a handset,” a “cell phone” or variants thereof. Furthermore, as used herein, the term “satellite” includes one or more satellites at any orbit (geostationary, substantially geostationary, medium earth orbit, low earth orbit, etc.) and/or one or more other objects and/or platforms (e.g., airplanes, balloons, unmanned vehicles, space crafts, missiles, etc.) that has/have a trajectory above the earth at any altitude. Finally, the term “base station” includes any fixed, portable or transportable device that is configured to communicate with one or more wireless terminals, and includes, for example, terrestrial cellular base stations (including microcell, picocell, wireless access point and/or ad hoc communications access points) and satellites, that may be located terrestrially and/or that have a trajectory above the earth at any altitude.
Diversity combining techniques, involving multiple spaced-apart transmit and/or receive antennas, are playing an increasing role in modern wireless communication systems, in particular 3G and 4G cellular systems. While receive diversity combining has been used for many years in different forms of radio communication, more recently, transmit diversity has also been gaining in popularity.
Transmit diversity systems are generally more complex than receive diversity systems. One reason for this complexity is that, in a receive diversity system, one can collect physically separate sets of diversified samples from physically separate (i.e., spaced apart) antennas, and the samples can then be combined according to different signal processing techniques to meet desired goals, such as increasing or maximizing signal-to-noise ratio, reducing or minimizing mean squared error relative to a pilot signal, etc. In contrast, a transmit diversity system is configured to launch signals from multiple spaced-apart transmit antennas such that, at a receive antenna, the received signal quality, after signal processing, provides an improvement over that which would be obtained with a single transmit antenna. Obtaining separable copies of the channel signals at the receiver is generally more challenging for transmit diversity.
Transmit diversity techniques can be categorized as follows: (i) co-frequency signals are transmitted simultaneously from multiple transmit antennas such that the signals are separable at a receiver signal processor, the separated copies being combined according to a chosen optimization criterion (the so called “Alamouti” method); (ii) achieving separability at the receiver through frequency diversity in the transmit signals; and (iii) switched transmit diversity, where the transmit signal is selectively transmitted from one of a multiplicity of antennas. In switched transmit diversity, the transmit antenna is selected which, it is anticipated, will offer a superior channel to the receive antenna.
While techniques (i) and (ii) may offer better performances than (iii), both (i) and (ii) generally use two or more separate transmit chains, depending on the order of the diversity system. While this may not be an excessive burden in a base station, it may be much more of a burden in a wireless terminal, which may be limited in form factor and/or battery power.
Therefore, switched transmit diversity may be favored in the return links (wireless terminal to base station) of cellular and mobile satellite communication systems. In a Time Division Duplex (TDD) system, in which forward and return links can use the same frequencies at different times, strong correlation generally exists between forward and return links if the TDD frame duration (a period of time encompassing at least one cycle of forward and return transmission) is sufficiently small. In contrast, in a Frequency Division Duplex (FDD) system, in which forward and return links use different (spaced-apart) frequencies, the frequency separation between forward and return links may be too great to provide high correlation between the forward and return channel transfer functions, when the transfer function is based exclusively on a multipath profile.
The above-described difficulty in using switched transmit diversity in FDD systems is illustrated in FIGS. 1A and 1B. In FIG. 1A, a wireless terminal 110, including two spaced-apart antennas 112 and 114, communicates with a terrestrial base station 120. In FIG. 1B, a wireless terminal 110, including two spaced-apart antennas 112 and 114, communicates with a satellite 130. It will be understood that FIGS. 1A and 1B may illustrate two separate terrestrial and satellite radiotelephone systems, a dual mode system in which a given wireless terminal 110 can communicate with a terrestrial base station 120 using terrestrial frequencies in a first mode and with a satellite 130 using satellite frequencies in a second mode, or a hybrid system in which satellite frequencies are used or reused for terrestrial communications with a terrestrial base station 120, which may also be referred to as an Ancillary Terrestrial Component (ATC). These hybrid systems are described, for example, in U.S. Pat. Nos. 7,636,567; 7,636,566; 7,634,234; 7,623,867; 7,603,117; and 7,418,263, that are assigned to the assignee of the present application, the disclosures of which are hereby incorporated herein by reference in their entirety as if set forth fully herein.
Referring to FIGS. 1A and 1B, for a given multipath profile, which is described mathematically by the impulse response of the channel, h(t), the channel frequency response, H(ω) (which is the Fourier transform of h(t)), may vary widely between the transmit and receive frequencies, even when h(t) is substantially invariant between the forward and return paths. Furthermore, it is noteworthy that, in some cases, h(t) may vary between the transmit and receive frequencies, e.g. when the multipath reflectivity is frequency dependent. For example, assume that Hfwd,1(ω1) and Hfwd,2(ω1) were the observed channel frequency responses in the forward direction from one terrestrial and/or satellite base station antenna to two receive antennas 112 and 114 on a given FDD wireless terminal 110, at the forward FDD frequency, ω1. Assume further that Hret,1(ω2) and Hret,2(ω2) were the corresponding return channel frequency responses at the return FDD frequency, ω2. Typically, Hfwd,1(ω1) is not equal to Hret,1(ω2) and Hfwd,2(ω1) is not equal to Hret,2(ω2), even when hfwd,1(t)=hret,1(t) and hfwd,2(t)=hret,2(t); this is due to separation of the forward link frequency and the return link frequency. Therefore, in an FDD system, as per present practice, a particular return channel is typically not selected for switched diversity return link transmission by measurements on a set of forward channels.
In summary, for typical duplexing frequency differences, the superior channel in the forward direction will not always be the superior channel in the return direction, when the channel differences are caused primarily by the multipath profile. An implicit assumption in this practice is that the mean antenna gain, averaged over angles-of arrival relevant to the propagation scenario, are similar. This has led to the practice of not using forward-link channel estimation to determine return antenna selection in transmit diversity systems.